// THIS DOES NOT WORK. IT WILL SKIP DATA:
uint8_t SPI_SlaveReceive(void)
{
    while(!(SPSR0 & (1<<SPIF0)));
    return SPDR0;
}

// THIS WORK.
// READ THE REGISTER 1st
uint8_t SPI_SlaveReceive(void)
{
    uint8_t data = SPDR0;
    while(!(SPSR0 & (1<<SPIF0)));
    return data;
}